Hybrid circuits based on copper and compatible resistors should be encapsulated to insure resistor durability in humid atmospheres. Furthermore, manufacturers prefer glass encapsulation to protect the copper from long-term corrosion. Only contact pads are soldered.
The encapsulant system must exhibit several features which are difficult to achieve together. It must form a bubble-free seal at low enough firing temperature to prevent shift of the underlying resistors. If the glass flows too much, it will diffuse into the resistor and shift the value upward. If it does not flow enough, it will not seal. The organic vehicle necessary for screen printing must burn out in nitrogen at this low temperature and the surrounding copper must remain stain-free to allow solder acceptance. Copper will form an oxide stain at low temperature which will disappear when fired at higher temperature (.about.600.degree. C.) in nitrogen.
Thus an ideal encapsulant should screen print smoothly and rapidly with a vehicle which is decomposable in a low oxygen-containing atmosphere such as nitrogen at a low enough temperature to allow the glass to flow sufficiently to form a seal, but not so much as to shift the resistors. Furthermore, the firing temperature should be high enough to remove any oxide which forms on the copper conductors.
A previous attempt to combine these features was the subject of U.S. patent application Ser. No. 012,133, filed Feb. 6, 1987 in which Pb(NO.sub.3).sub.2 was used to coat low melting glass and burn out the vehicle at low firing temperature. The low melting glass formed a good seal without excessive resistor shift. A problem with this approach is, however, that excess oxidizing agent [Pb(NO.sub.3).sub.2 ] required for complete vehicle removal and bubble-free film formation may also oxidize the copper and thus form an unsolderable stain. On the other hand, higher temperature firing which would remove the stain causes excess glass flow and consequently resistor shift.